1. Field of the Invention
The present invention pertains generally to linear mechanical screw drives for moving a load back and forth between two end-of-travel positions, such as in a door or gate actuating system, where the load is pulled by a load carrier element threaded on a rotary drive shaft between two unthreaded sections of the shaft. This invention improves the load carrier to resist a damaging vibration following disengagement of the load carrier at the unthreaded shaft sections as the carrier is urged by a biasing spring against the thread end of the rotating drive shaft. This invention also provides the drive shaft with oversized thread surfaces at the thread ends for enhanced wear resistance.
2. State Of The Prior Art
Linear mechanical drives of the screw type find wide application and are commonly adapted for opening and closing doors and sliding gates, such as remotely operated garage doors. One particular application for screw drives has been the remote actuation of roll-up type truck loading doors on cargo van and trailer truck doors. Roll-up doors are made of several panels hinged together along their horizontal edges and held between vertical slide tracks in a door opening, for holding the hinged panels in a flat vertical plane to close the door opening. The upper ends of the slide tracks curve to a horizontal position above the door opening such that pushing up on the door successively brings the panels to a horizontal out of the way position. Conventionally, such door actuating drives have included a worm gear or screw shaft mounted to the ceiling of the van/trailer cargo enclosure and driven by a reversible motor powered by the vehicle battery. A nut threaded on the screw shaft is displaced axially by rotation of the screw. The roll-up door is connected to and pulled by the nut between the opposite ends of threaded drive shaft. This general type of linear drive is well known and widely used. Difficulties have been encountered, however, in applications requiring precise positioning of the load at one or both ends of the drive shaft. Rotary inertia of the drive motor introduces a positioning error in systems which rely on timers or load position sensors to activate and deactivate the drive motor. More sophisticated systems capable of electronically sensing and accurately positioning the load are costly and require more complex installation wiring of the system. In applications such as truck door and garage door actuators it is desirable to minimize the cost and complexity of the system without, however, sacrificing reliability. A continuing need exists for simple drive systems capable of long term reliability and load positioning accuracy with minimal maintenance, particularly in difficult environments such as cargo compartments of transport vehicles where the drive system is exposed to severe vibration, shock, ambient temperature extremes, humidity and moisture.
U.S. Pat. No. 4,821,456 issued Apr. 18, 1989 to this applicant for a Linear Mechanical Drive With Precise End-of-Travel Load Positioning, disclosed a mechanism featuring a drive shaft with a male threaded shaft section intermediate two smooth unthreaded shaft sections. The drive shaft is mounted to a supporting structure and is turned by a reversible motor drive. An internally threaded load carrier unit is axially displaceable along the drive shaft from one to another of the two unthreaded shaft sections in response to rotation of the drive shaft. The load carrier disengages from the shaft thread at each of the unthreaded sections to positively stop axial movement and precisely determine the end-of-travel positions of the carrier and any load connected to the same, irrespective of continued drive shaft rotation. The load carrier in its disengaged condition at each unthreaded section of the shaft is spring biased into contact with the drive shaft thread, to maintain the load carrier ready for re-engagement with the shaft thread when rotation of the shaft is subsequently reversed, to then pull the load back in the opposite direction along the drive shaft.
Because of inertia inherent in the motor drive which turns the threaded shaft, the shaft usually continues to rotate for some time following disengagement of the load carrier unit at one or the other of the unthreaded sections of the shaft. When this occurs the load carrier vibrates, reciprocating back and forth a short distance along the drive shaft. This happens because with each revolution, at a certain relative angular position between the male and female thread ends, the end of the male thread on the shaft is free to advance about a quarter pitch into the female thread of the load carrier, and the spring bias drives the load carrier against the end of the male thread. As the shaft turns the male thread then withdraws from the female thread. This process repeats with each revolution of the shaft and causes undesirable vibration and damage to the parts of the mechanism. As the thread pitch increases, so does the distance by which the shaft thread is able to penetrate the carrier, and the greater the impact of the load carrier against the end of the shaft thread. This spurious vibration becomes particularly severe in the large pitch thread drive shafts used in door openers in trucks and vans, garages, and the like.
One prior solution to this difficulty, described by this applicant in the aforementioned '456 patent, consists of a thread follower attached to the load carrier which remained in engagement with the shaft thread and also maintained the load carrier spaced from the end of the shaft thread following disengagement of the load carrier. The load carrier was thus unaffected by continued rotation of the drive shaft, yet was pulled into re-engagement by the thread follower once the drive shaft was turned in the opposite direction. While this approach works well, it introduces some vibration into the system because the thread follower is a spring which snaps over the shaft thread with each revolution of the shaft.
Further improvement is desirable to prevent or minimize spurious reciprocal motion and vibration of the load carrier against the shaft thread, and to reduce wear on both the load carrier and the shaft thread for maintaining precise load positioning and extending the service life of the mechanism.